According to Gazzetta dello Sport, Daniele Orsato was well within his rights to add extra time to the minimum of five of injury time due to other stoppages.
“AC Milan equalise in the 97th minute, while it had been reported that only five minutes would be played. Error? No, in fact the length of time has only been lengthened by one minute,” the paper says.
“The substitution of Jonathan Biabiany in the 91st minute and the game was resumed in the 92nd. Then came the booking of Manuel Locatelli in the 95th minute and it then took another minute to resume play.
“The referee then decides, as his faculty, to extend injury time. Milan then win a corner and we were only in the seventh minute of injury time by 15 seconds. Orsato suggests that the end of the game would come at the end of this action, which coincided with Zapata scoring with the last kick of the game.”
Source: Gazzetta dello Sport